The invention relates to a hydrostatic auxiliary power steering mechanism for motor vehicles. This type of auxiliary power steering mechanism is generally known from DE-3037206 A1 and from FIG. 3 of DE-OS 2058731.
Conventional hydrostatic auxiliary power steering mechanisms of the type described above are provided with a servo mechanism acting as a power amplifier for the steering mechanism. For this purpose, there is incorporated into the servo mechanism a steering motor, for example, in the form of an orbit hydromotor which, in conjunction with an upstream servo steering valve, can conduct servo hydraulic fluid to the steering cylinders. The steering valve and steering motor together constitute one steering unit, an example of which is shown in FIG. 1, to which reference is now made. The steering motor is designated in this figure by the reference symbol 2 and the servo steering valve by the reference symbol 4. A line 6 carrying servo hydraulic pressure P is conducted through the continuously adjustable distributing valve as servo steering valve 4 and discharged into tank or reservoir T in the neutral position in accordance with FIG. 1. As a function of the turning motion [psi].sub.w, a corresponding position of the servo valve 4 is obtained, as delayed by spring 8, whereby the line 6 is switched onto one of the two working lines L or R by the steering motor 2. The steering valve 4 and the steering motor 2 are thus a component of a servo control loop which meters a hydraulic stream QV to a steering cylinder. This relationship becomes clearer in FIG. 2, which represents a block diagram of a conventional hydrostatic auxiliary power steering mechanism. The steering wheel turning angle should be regarded as the command variable [psi].sub.w. The actual turning angle of the steering motor is designated as [alpha].sub.x. This creates an internal feedback loop for the steering unit which consists of a steering motor and a steering valve, whereby errors in the transmission signal can be controlled.
When the hydraulic energy supply fails, i.e., when the servo pressure P, for example, falls to zero, the steering motor 2 in the conventional hydrostatic auxiliary power steering mechanism is converted to a hand pump driven by the steering wheel with which the steering cylinders can be fed through the lines L and R.
Aside from the fact that the possibilities for optimization are limited in customary hydrostatic auxiliary power steering mechanisms due to the close connection between signal flow and energy flow, it has turned out to be particularly disadvantageous that the steering forces are often too great when the servo mechanism fails. The system-related reason for this is that an adjustment of the servo valve 4 to the steering motor 2 must be undertaken in order to provide optimal servo operation. However, this means that the power transmission between the steering wheel motion and the steering gear is established and cannot be changed.
An object of the invention, therefore, is to develop a hydrostatic auxiliary power steering mechanism such that, with a servo mechanism optimally adjusted to the steering system, it is guaranteed that even the heaviest vehicles remain easily steered even in the event of failure of the servo mechanism.
By means of the invention, the transmission ratio for the emergency steering function can be designed so that it is independent of the control loop of the servo mechanism, using the hand pump driven by the steering wheel. The control and design engineers thereby gain more flexibility in the design of the hydrostatic auxiliary power steering mechanism, so that optimal conditions for the achievement of a desired steering characteristic can be created. For example, the dynamic and static transmission behavior of the steering system can be adapted to comfort and maneuverability requirements, free from external pressures, using the controller which is isolated from the hand pump. This results in the additional advantage that the control system can be expanded so that a larger number of disturbance parameters can be taken into consideration in the control process. This results in further advantages in terms of construction. For one thing, only the hand pump with two hydraulic lines is still located in the vicinity of the driver's seat, whereas in the state of the art, the hydraulic energy with four or five pipes or fluid conduits had to be laid near the driver's seat. Because the auxiliary hydraulic energy can now be conducted directly to the steering cylinders, the integration of the hydraulic components into a system with multiple users is simplified. Finally, the system is adaptable to extensive integration of standard elements into existing hydraulic systems, for example, the use of proportional valves. Although the principle of connecting a hand pump driven by the steering wheel in parallel to the auxiliary pump driven by a motor is known (DE 2655379 A1), the steering valve controlling the hydraulic auxiliary energy supply is located downstream of the connection of the auxiliary pump line and the hand pump line so that the steering valve always remains active, i.e., even during hand steering operation. This results necessarily in detectable differences in the steering response as a function of whether the servo mechanism is active or inactive.
Accordingly, all disturbance variables affecting the control system between the steering valve and the steering linkage can be controlled.
With the switchgear provided between the steering valve and the steering cylinders, the hand pump driven by the steering wheel can be in constant contact with the control system. When the servo mechanism fails, therefore, the regulating signal of the hand pump is immediately available, so that the vehicle can be reliably steered in every phase of operation. This was not possible prior to the parallel connection of the steering valve and hand pump in accordance with the present invention, whereby the hand pump is thus essentially running continuously in the background of the servo mechanism.
When a microcontroller is used as the controller, additional advantages result in terms of operational safety of the steering system. This kind of microcontroller is not only capable of executing the actual control algorithm but can also execute safety and diagnostic algorithms. Moreover, additional reference values or auxiliary control variables can be supplied to such a microcontroller, which makes it possible to easily adapt the steering behavior to the driving conditions.
The circuit element of the hydrostatic auxiliary power steering mechanism according to the invention opens up the possibility of integrating into the hydraulic system a preferably proportional action servo steering valve. The invention also contemplates that the emergency steering function using the hand pump takes over only when the pressure in the auxiliary line is greater than that in the corresponding working line. Moreover, return valves prevent the steering wheel from turning under the influence of the pressure in the steering cylinder.
The invention further contemplates that the auxiliary lines are always adequately supplied with pressurized fluid so that the immediate response of the emergency steering is guaranteed.
The pump which supplies hydraulic fluid to the servo mechanism may be a pump with fixed displacement. It is also possible, however, to retain the servo steering valve as a standard element and to work with a regulated or variable displacement pump, as is the case with load-sensing circuits. In this case, it is advantageous to design the servo valve as a hydraulically pilot-controlled load-sensing servo valve and to conduct the load status line through a pilot valve which either discharges the load status line into the tank or closes it, creating a load pressure signal, depending on the level of pilot pressure, which is preferably formed by a pressure regulating valve with three controlled connections.
It is advantageous to conduct the load status signal to a return valve logic, which switches on the emergency steering function if the load status signal falls below a certain predetermined value. For example, this kind of return valve logic can be in the form of two inverted shuttle valves connected in parallel to which from one side pressure is admitted from the load status line and on the other side from the auxiliary lines and which control the discharge to the tank. In this way, the pressure in the auxiliary lines is reduced below a certain threshold in the presence of a load status signal.
In place of this return valve logic, it is also possible to use a pilot valve which is triggered directly by the pilot pressure of the load-sensing servo valve.
The invention also considers a servo steering valve, in which the working line not under servo pump pressure can be switched to tank or reservoir using this valve. A simplified arrangement of the servo steering valve is achieved when the return valves integrated into the working lines upstream of the openings of the accompanying auxiliary pressure lines prevent a pressure drop in the working line from occuring through the servo steering valve during emergency steering operation.
Further, during pure hand pump operation, the oil displaced into the working line loaded with the least pressure can flow to the tank.
In order to eliminate the effect on steering of malfunctions, whether electronic or hydraulic, in the controlled auxiliary power, it is advantageous to integrate into the working line upstream of each return valve a hydraulically pilot-controlled return valve to which pressure is admitted from the direction of the steering valve in the direction of closing and whose accompanying control pressure line is connected to the side of the hand pump, through which the relevant working line can be fed. Using these two hydraulically pilot-controlled return valves, the hydraulic auxiliary energy is always decoupled or inoperative when the support direction and the steering direction given by the hand pump do not agree.
By the use of highly disturbance-proof return valves, a pressure divider circuit is created which allows a load pressure reduced in constant proportion to the working pressure to become active on the hand pump driven by the steering wheel. A reaction feeling is thus constantly transmitted to the driver of the vehicle even in the servo-supported steering operation, reflecting the forces working on the wheels. A further advantage of this arrangement is to be viewed in the fact that the hydraulic resistors, i.e., screens, are arranged in such a way that complete or partial closure cannot trigger safety problems but rather, at the worst, can lead to sacrifices in comfort.
Further simplifications of the safety circuit provided for the auxiliary power steering result from the developments of the invention.